The invention concerns a device for blending dust, powder and coarse particle bulk materials.
The various types and function means of blending devices are described in the DE-A1No. 35 12 538. The content of this publication is hereby referred to expressly.
During the blending process of bulk materials a so-called "funnel flow" or "mass flow" sets in. Mass flow occurs when the total bulk material during discharge starts to move like a fluid, i.e. the total hopper cross-section falls regularly. In the case of funnel flow the material only flows in a "channel" which extends vertically above the outlet and also has approximately the same diameter. At the upper end of the channel a crater shaped depression forms, on the surface of which the material slides radially towards the outlet. Outside this flow crater and outside the flow channel a so-called "dead zone" is formed during the discharge process of a funnel flow hopper, in which no particle movement takes place. A blending effect as in the case of mass flow does not set in. Funnel flow always sets in when the slope angle of the hopper above the material drain outlet is not steep enough. The selection and positioning of the discharge hopper therefore plays a critical role in producing the desired mass flow according to the material characteristics. Various measures for obtaining a best possible blending effect are described in the DE-A1No. 35 12 538.
A further device has become known from the DE-ASNo. 1 507 885 with which a constant blend of the material is apparently achieved without the necessity of returning the material to the silo via a return pipe. This is apparently achieved by providing each gravity pipe with several inlet openings at different heights, whereby each gravity pipe is kept full up to its upper inlet opening underneath the material level in the silo. In the case of this known device the material flows through the gravity pipe at a lower velocity than that of free gravitational velocity, which results in the fact that only the upper inlet openings in the gravity pipe situated in a material layer of the silo are effective, while material is not able to enter the inlet openings below, as the gravity pipe is full in this area. The distribution of the inlet openings in the gravity pipe is formed in such a way that geometrically similar sections of the material in the silo are connected with an opening, so that the material is drained regularly through the openings and a constant blend of the material levels in the silo is achieved, independant of the blended material level of the material. If the blended material level falls gradually, the geometrically similar sections or volumes of the material to be blended is reduced proportionally. The content of this publication DE-AS-1 507 885 is referred to expressly.